The present application relates generally to the field of lithium-ion batteries or cells. More particularly, the present application relates to improved solid-electrolyte inter-phase (“SEI”) layer formation on a negative electrode of a lithium-ion battery.
Lithium-ion batteries or cells include one or more positive electrodes, one or more negative electrodes, and an electrolyte provided within a case or housing. Separators made from a porous polymer or other suitable material may also be provided intermediate or between the positive and negative electrodes to prevent direct contact between adjacent electrodes. The positive electrode includes a current collector having an active material provided thereon, and the negative electrode includes a current collector having an active material provided thereon. The active materials for the positive and negative electrodes may be provided on one or both sides of the current collectors.
FIG. 1 shows a schematic representation of a portion of a lithium-ion battery 1 such as that described above. The battery 1 includes a positive electrode 10 that includes a positive current collector 12 and a positive active material 16, a negative electrode 20 that includes a negative current collector 22 and a negative active material 24, an electrolyte material 40, and a separator (e.g., a polymeric microporous separator, not shown) provided intermediate or between the positive electrode 10 and the negative electrode 20. The electrodes 10, 20 may be provided as relatively flat or planar plates or may be wrapped or wound in a spiral or other configuration (e.g., an oval configuration). The electrodes may also be provided in a folded (e.g., a zig-zag, accordion, or z-fold) configuration.
During charging and discharging of the battery 1, lithium ions move between the positive electrode 10 and the negative electrode 20. For example, when the battery 1 is discharged, lithium ions flow from the negative electrode 20 to the positive electrode 10. In contrast, when the battery 1 is charged, lithium ions flow from the positive electrode 10 to the negative electrode 20.
Once assembly of the battery is complete, an initial charging operation (referred to as a “formation process”) may be performed. During this process, one goal is to form a stable solid-electrolyte-inter-phase (SEI) layer at the negative electrode and also possibly at the positive electrode. These SEI layers are formed through a process of irreversible reactions between the electrodes and the electrolyte (e.g., with decomposing solvents of the electrolyte). Beneficially, these SEI layers act to passivate the electrode-electrolyte interfaces as well as to prevent side-reactions thereafter, thereby increasing battery stability.
However, there may also be several downsides associated with SEI layers. For example, SEI layers are formed with lithium that would otherwise be cyclable. Thus, to achieve a desired capacity or cell balance (i.e., balance between capacity of the positive electrode to absorb lithium), additional positive active material would be required, which could increase cost, increase weight, and decrease power density of the battery. The SEI layers also have insulating properties that may result in impedance or resistance within the battery, which can negatively impact the charge and discharge characteristics of the battery. The SEI layers may also continue to grow in thickness over the course of a battery's life, which further removes cyclable lithium from the battery, thus decreasing capacity and increasing resistance or impedance. The SEI layer may also degrade over the life of the battery, such as when the electrode shrinks and expands due to heat associated with charging or discharging, which may cause cracks or other irregularities may form in the SEI layer. At the location of these irregularities, new SEI will be formed by a reaction that further reduces the amount of cyclable lithium.
One possible manner to address the problems associated with degradation of the SEI layer is to initially create thicker, more robust SEI layer. However, this would require more lithium at formation, which would either reduce battery capacity or require additional positive active material as described above. Further, as the amount of lithium required to form a thicker SEI at the negative increases, the voltage of the positive active material must be increased to deliver the lithium. This may, in some circumstances, affect overall stability of the positive electrode and related cell components due to parasitic reactions occurring at higher voltages, which may lead to capacity fade and impedance increase. Also, because SEI formation is known to best occur in a slow, controlled process, a thicker SEI layer would require a longer manufacturing or formation time.
Accordingly, it would be advantageous to provide a lithium ion battery with improved SEI formation.